Polystyrene substrate provided with a polyurethane coating

ABSTRACT

A single-layer two-component polyurethane lacquer which is particularly suitable for the high-gloss coating of polystyrene. A lacquer which can be sprayed having a potlife of at least 16 hours and a flash point exceeding 21° C. is obtained by using tertiary alcohols and diacetone alcohol as a solvent, to which a thermoplastic acrylate resin is added as an adhesive.

This is a division of application Ser. No. 07/784,726 filed Oct. 28,1991, now abandoned, which is a continuation of application Ser. No.07/550,249 filed Jul. 6, 1990, now abandoned, which is a continuation ofapplication Ser. No. 07/345,400 filed May 1, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The invention a method of lacquer relates to a method of lacquerpolystyrene-containing substrates, provided with high-gloss coatings ofpolyurethane.

Industrial applications of polystyrene are, among others, supports forhousehold appliances, housings for audio and video equipment, shaversand wheel covers for cars. Polystyrene is often used in the form ofso-called high-impact polystyrene, in which a few percent of rubber aremixed with the polymer. For many of these applications it is requiredthat the surface of the polystyrene is provided with a hard, properlyadhering decorative coating. This coating can be obtained by applying alacquer, preferably, by spraying.

Physically drying lacquers are not very attractive because of their highcontent of organic solvents. Owing to tightened environmentalrequirements the emission of solvents has to be minimized. Also surethere is no crosslinking in physically drying lacquers, the resistanceto wear and the chemical resistance of the coating manufactured by meansof these lacquers are often insufficient.

Lacquers which are more suitable are two-component lacquers, for examplepolyurethane lacquers, which are cured by means of crosslinking andwhich contain relatively few organic solvents.

Polystyrene is attacked by many common solvents, such as ketones andethers, so that the object made from polystyrene is subject todeformation and the surface is attacked in such a way that it attains anundesirably mat appearance.

A polyurethane lacquer on the basis of polyhydroxyacrylate andpolyisocyanate is known from European Patent Application no. 1304. Thelacquer described therein contains in addition to a tertiary alcoholalso 18-60% by weight of one or more other solvents such as ketones orethers. This lacquer is unsuitable for use on polystyrene, because thelatter solvents attack the polystyrene, which leads to theabove-mentioned disadvantages. Additionally, the lacquer describedtherein contains no adhesion-improving substance so that for manyapplications a primer has to be provided first on the substrate, whichimplies an additional process step. This is generally referred to as atwo-layer paint system.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome the above-mentioneddisadvantages and provide a single-layer two-component polyurethanelacquer having a potlife of at least 16 hours, which can suitably beused for applying a properly adhering coating to polystyrene, and,moreover, which can be applied by means of spraying.

This object is achieved in accordance with the invention by apolyurethane lacquer as described in the opening paragraph,characterized in that the lacquer also contains diacetone alcohol and anadhesion-improving substance. Solvents which do not attack polystyreneare aliphatic hydrocarbons and alcohols. Aliphatic hydrocarbons are notsuitable because polymeric components of the lacquer do not dissolve insuch solvents. Primary and secondary alcohols react with polyisocyanateand, thus, block the formation of polyurethane. Consequently, lacquerscomprising these alcohols must be sprayed soon after they have beenprepared. Tertiary alcohols do not react with polyisocyanate because ofsteric hindrance of the hydroxyl group by the alkyl groups andconsequently they can suitably be used as a solvent for a polyurethanelacquer for polystyrene. In addition to a tertiary OH group, diacetonealcohol (4-hydroxy-4-methylpentanone-2) also contains a ketone group.This ketone group increases the solubility of the polyhydroxyacrylateand the polyisocyanate and also has the favourable property that, unlikeother ketones, it does not attack polystyrene. Moreover, diacetonealcohol has the advantage that it evaporates more slowly than othertertiary alcohols, so that during and after its application onto asubstrate the lacquer exhibits a better levelling. In order to increasethe solubility of the polyhydroxyacrylate and the polyisocyanate,low-molecular polymers are used. A suitable low-molecularpolyhydroxyacrylate is marketed by the firm of Synthese under the tradename of Setalux, type 1752 X 65. A suitable low-molecular polyisocyanateis marketed by Bayer under the trade name of Desmodur, type N 3300. Inorder to obtain a desirable evaporation rate and hence a desirabledrying time, the lacquer comprises tertiary alcohols, such as tertiarybutanol or tertiary pentanol, in addition to diacetone alcohol. Thequantity of tertiary alcohol, unlike diacetone alcohol, ranges between25 and 60% by weight. If only diacetone alcohol is used as the tertiaryalcohol the drying time of the lacquer becomes unpracticably long. Inaddition to the said tertiary alcohols other tertiary alcohols mayalternatively be used, the maximum number of C-atoms being approximately15, provided that for practical reasons the melting point of thetertiary alcohol remains below 30° C. An additional and importantfurther advantage of the lacquer according to the invention is that theflash point can be formulated at a temperature above 21° C. A monolayertwo-component system which is very suitable for the high-glosslacquering of polystyrene can be obtained by adding anadhesion-improving substance, such as an acrylate resin, to thislacquer.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the polyurethane lacquer in accordance with theinvention is characterized in that the adhesion-improving substance is athermoplastic acrylate resin. These acrylate resins adhere well to manytypes of substrates, such as metals and synthetic resins, and can bedissolved in the polyurethane lacquer in accordance with the invention.

A preferred embodiment of the polyurethane lacquer in accordance withthe invention is characterized in that the thermoplastic acrylate resincontains polymethyl methacrylate. Such an acrylate resin is marketed bythe firm of Rohm under the trade name of Plexigum, type M 345.

An embodiment of the polyurethane lacquer in accordance with theinvention is characterized in that the lacquer contains at most 5% byweight of polymethyl methacrylate. A higher polymethyl methacrylatecontent adversely affects the gloss and the levelling of the lacquer.

A polyurethane lacquer in accordance with the invention is furthercharacterized in that the lacquer contains at least 5 and at most 17% byweight of diacetone alcohol. A higher content of these alcohols leads toundesirably long drying times due to the slow evaporation. Below 5% byweight of diacetone alcohol the levelling rate is too low.

A polyurethane lacquer in accordance with the invention is furthercharacterized in that the lacquer contains at most 10% by weight ofxylene and at most 15% by weight of butyl acetate. These solvents areoften necessary to increase the solubility of the polyhydroxyacrylateand the polyisocyanate. If smaller concentrations are used no attack ofthe polystyrene takes place. Ethyl acetate cannot be used as a solventif the flash point of the lacquer is to exceed 21° C.

A special embodiment of the polyurethane lacquer in accordance with theinvention is characterized in that the lacquer contains as a catalyst amixture of calcium naphthenate and dibutyl tin dilaurate in aconcentration of maximally 0.5% by weight. If such a catalyst is used,the drying time of the lacquer amounts to half an hour at 50° C. Ahigher concentration of the catalyst adversely affects the adhesion ofthe lacquer. To compensate for this, more thermoplastic acrylate resinmust be added to the lacquer as an adhesion-improving substance, yet, asstated above, the gloss and the levelling of the lacquer will beadversely affected. If the lacquer comprises no catalyst the drying timeamounts to many hours.

Levelling-agents such as, for example, Additol XL 490 marketed byHoechst, may further be added to the lacquer. Moreover, inorganic andorganic pigments may also be added to the lacquer, as well as fillersand pigment-dispersing agents. In order to obtain the so-called metalliclacquers, metal suspensions can be added, for example of aluminium.

A further object of the invention is to provide a spray diluent for apolyurethane lacquer in accordance with the invention.

This object is achieved in accordance with the invention by a spraydiluent, characterized in that the diluent contains a tertiary alcoholand diacetone alcohol. These solvents have the advantages and propertiesmentioned hereinbefore, namely, they do not attack polystyrene; they donot react with polyisocyanate and can have a flash point exceeding 21°C. if, for example, tertiary pentanol is used as a solvent.

A further object of the invention is to provide a method of lacquerspraying substrates containing polystyrene, in which a polyurethanelacquer and a spray diluent, both in accordance with the invention, aresprayed in the usual manner after they have been mixed.

The invention will now be explained in more detail by means of thefollowing exemplary embodiment:

The following solutions were prepared:

    ______________________________________                                        Solvent (solution A):                                                                76   kg of tertiary pentanol                                                  12   kg of butyl acetate                                                      12   kg of diacetone alcohol.                                          Catalyst solution (solution B):                                                      92.65                                                                              kg of solution A                                                         7    kg of calcium naphthenate                                                0.35 kg of dibutyl tin dilaurate.                                      ______________________________________                                    

Solution A and calcium naphthenate were mixed by stirring; subsequentlydibutyl tin dilaurate was added.

    ______________________________________                                        Adhesive (solution C):                                                        19 kg      of Plexigum M 345 (thermoplastic                                              methyl methacrylate resin)                                         17 kg      of butyl acetate                                                   59 kg      of solution A                                                      5 kg       of diacetone alcohol.                                              ______________________________________                                    

The solution was heated to 60° C. and stirred until the methacrglateresin was dissolved.

    ______________________________________                                        Spray diluent (solution D):                                                            44 kg of solution A                                                           28 kg of diacetone alcohol                                                    28 kg of butyl acetate.                                              ______________________________________                                    

The following clear coating was prepared:

    ______________________________________                                        65.5 kg Setalux 1752 XS 65 (polyhydroxyacrylate)                              26.4 kg of solution C (adhesive)                                              12.8 kg of diacetone alcohol                                                   4.0 kg of solution B (catalyst)                                               0.3 kg of Additol XL 490 (flow-promoting agent)                              ______________________________________                                    

100 parts by weight of this lacquer are mixed with 18 parts by weight ofDesmodur N 3300 (polyisocyanate). By means of 25 parts by weight ofsolution D, the viscosity of the lacquer was rendered suitable forspraying. Plexigum M 345 is marketed by the firm of Rohm; Setalux 1752XS 65 is marketed by the firm of Synthese; Desmodur N 3300 is marketedby Bayer and Additol XL 490 is marketed by Hoechst.

The lacquer viscosity for spraying was measured in accordance with DIN53211 by means of a DIN-cup no. 4 and amounted to 22 seconds (effluxtime). The flash point is >21° C.

The lacquer was sprayed, in a customary manner, on test boards of amoldflown high-impact polystyrene. After the spraying process the testboards were dryed at 50° C. for 30 minutes; subsequently, for 7 days at23° C. and a relative humidity of 50%. This drying time was selectedbecause in general a poor adhesion does not show until after a few days.

The results of the test are listed in the Table below:

    ______________________________________                                        layer thickness (in accordance with                                                                 30 μm                                                ISO 2808)                                                                     gloss (in accordance with                                                                           96                                                      ISO 2813)                                                                     adhesion (ISO 2409)   Gt-O                                                    resistance to ethanol better than 200 rubs                                    resistance to white spirit                                                                          better than 200 rubs                                    scratch resistance    satisfactory                                            hardness (Konig, in accordance                                                                      52 s.                                                   with ASTM 4366)                                                               art. sweat (2.6 DIN 53160)                                                                          RAP-O                                                   art. sweat (8.8 DIN 53160)                                                                          RAP-O                                                   potlife               20 hours                                                drying time:                                                                  sand dry (ASTM D 1640)                                                                              30 min                                                  touch dry             90 min                                                  "dried trough"        240 min                                                 adhesion after dry heat test                                                                        Gt-O                                                    (IEC 68-2-2B6)                                                                adhesion after cold storage                                                                         Gt-O                                                    (IEC 68-2-1-AB)                                                               ______________________________________                                    

The adhesion was measured by means of the so-called diamond scratchingtest (Gitterschnitt) in which a square pattern is scratched into thelacquer and an adhesive tape is provided on the scratches andsubsequently removed. The number of lacquer squares which is removedfrom the substrate is a measure for the adhesion. If all squares remainon the substrate the adhesion is very good (Gt-O). The resistance tochemicals was determined by rubbing with a cloth immersed in therelevant substance. After rubbing 200 times no attack of the lacquercould be established. The resistance to scratching was determined bymeans of the so-called nail test (mar resistance).

I claim:
 1. A polystyrene substrate provided on the surface thereof witha high-gloss coating of a polyurethane forming lacquer, said lacquercomprising 25-60% by weight of a tertiary alcohol selected from thegroup consisting of tertiary butanol and tertiary pentanol, 5-17% byweight of diacetone alcohol, a polyhydroxyacrylate, a polyisocyanateand, as an adhesion improving substance, a thermoplastic polymethylmethacrylate.
 2. The polystyrene substrate of claim 1 wherein thelacquer contains a maximum of 5% by weight of polymethyl methacrylate.3. The polystyrene substrate of claim 1 wherein the lacquer contains0-10% by weight of xylene and 0-15% by weight of butyl acetate.
 4. Thepolystyrene substrate of claim 1 wherein the lacquer contains acatalyst.
 5. The polystyrene substrate of claim 3 wherein the lacquercontains a mixture of calcium napthenate and dibutyl tin dilaurate as acatalyst in a concentration of maximally 0.5% by weight.